We have purified rabbit lucocyte pyrogen and have raised an antibody to it in goats. With this antibody we propose (1) to confirm by gel diffusion and immunoelectrophoresis that the pyrogen is in fact homogeneous (2) by the use of fluorescent antibody, to locate pyrogen within phagocytic cells. (3) To see whether monocyte pyrogen is also neutralized by anti-leucocyte pyrogen antibody. We are currently trying to label purified leucocyte pyrogen with I125, and alternatively with 3H formaldehyde. If either of these attempts is successful, we will develop a radioimmunoassay which hopefully can be used to measure pyrogen in various body fluids and tissues. Since human pyrogen causes fever in rabbits, it may be that antibody to rabbit pyrogen will also bind to human pyrogen. In this case it would be possible to use the radioimmunassay to study pyrogens in human diseases. When enough pyrogen can be spared from the above projects, we hope to confirm homogeneity using the analytical ultracentrifuge, and to establish its primary structure.